The present invention relates to multi-spectral image sensing, and in particular to the monolithic integration with CMOS of light-sensing devices tailored for different spectral ranges, such as UV, Visible, SWIR, MWIR, LWIR, etc.
Co-pending patent application Ser. No. 11/070,721 introduced a method of fabricating, on the same substrate and monolithically integrated with CMOS, light-sensing devices capable of covering different portions of the electromagnetic spectrum, as shown in FIGS. 1A and 1B (Prior Art). Both types of devices comprise the same set of epitaxially layers, with in-situ doping and bandgap engineering, grown on separate active areas with complementary doping polarities.
A first device type, the PIN type, is a two-terminal device wherein a first electrode is p-type doped, and a second electrode is n-type doped, with an undoped or lowly doped between the two electrodes, forming a P-I-N structure. In PIN devices light is absorbed mainly through band-to-band transitions of charge carriers.
A second device type, the HIP type, is also a two-terminal device wherein the two electrodes are doped with impurities of the same type, forming wither P-I-P or N-I-N structures. In HIP devices, light is absorbed mainly through transitions within the same band (within the conduction band or within the valence band), wherein the energy levels may be quantized or not. Examples of such devices with quantized energy levels are Quantum Well Infrared Photodetectors (QWIP), and examples of devices without quantized are Heterojunction Internal Photoemission (HIP) devices.
The epitaxially grown layers comprise a region of undoped or lowly doped layers, and a region that is an electrode of the light-sensing devices. For epitaxial layers with in-situ p-type doping, the films grown on n-type active areas form P-I-N structures, and the same films grown on p-type active areas form P-I-P structures. For epitaxial layers with in-situ n-type doping, the films grown on p-type active areas form P-I-N structures, and the same films grown on n-type active areas form N-I-N structures.
The ranges of wavelengths absorbed by either type of device depend on the details of the in-situ doping and bandgap engineering of the epitaxial layers. When using SiGeC-based epitaxial films, the band structure of the light-absorbing layers is determined by the amount of Ge and C, and the atomic-level ordering of the incorporation of said elements. Said band structure determines the ranges of wavelengths possible to absorb in each type of device. Bandgap engineering and doping profile engineering of the light-absorbing layers, can also be used to place photo-generated carriers under a built-in drift field, which is crucial to insure a highly efficient charge collection and readout processes.
Said co-pending application Ser. No. 11/070,721 provided several exemplary implementations using different types of substrates, bulk and SOI, and for image sensors having front-side illumination as well as for image sensors having back-side illumination, as shown in FIGS. 2A and 2B (Prior Art). As depicted in this exemplary implementation, at the opposite end from which light is coupled into the device, the junction/electrode can be a Schottky junction.
In addition, both types of devices can have an avalanche region placed between their top and bottom electrodes, thereby providing a built-in gain mechanism, which can be bandgap engineered as shown FIGS. 3A and 3B (Prior Art).
Co-pending patent application Ser. No. 11/070,721 also introduced a method of fabricating Heterojunction Integrated Thermionic (HIT) coolers, with the light-sensing layers, through the same epitaxial growth step. The HIT layers can be grown before or after the layers for light-sensing, leading to different integration and interconnecting schemes, as shown in FIG. 4A (Prior Art).
In addition, both types of devices can have an energy-filtering and/or momentum-filtering region placed between their top and bottom electrodes, as shown FIG. 5 (Prior Art), as described in WO/2006/010618.